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Numerical study of local scour effects on the lateral pile-soil interaction
Get PDFLocal scour usually occurs around offshore pile foundations, which may bring the reduction of both the effective stresses in the surrounding soil and the effective pile embedment. Until now, the effects of local scour on the lateral pile-soil interaction have not been well understood. In this study, a 3-D finite element model for the lateral pile-soil interaction is proposed and verified with experimental results. An improved analysis approach is further presented for evaluating the local scour effects on the lateral pile-soil interaction by extracting p-y curves from the numerical results. The results indicate that scour could induce a significant transition of pile behavior for a monopile typically employed for offshore wind turbines. If the pile exhibits rigid structural behaviors after scour, the pile’s tip and shaft resistance may become a significant component of the whole soil resistance, which would render the traditional p-y approach inapplicable. For a given depth below the scour base, the p-y curves get significantly stiffer with increasing scour depth, especially at relatively shallow depths. The scour effects on the variation of p-y curves from the present numerical results are generally consistent with the existing centrifuge results. As the slope angle of scour holes decreases, the effect of the remaining sloping overburden soil above the level of the scour base reduces and the stiffening of the p-y curves at a given depth below the scour base is correspondingly alleviated
Scour protection design in highly morphodynamic environments
Get PDFOffshore wind parks are often developed in areas with a highly morphodynamic seabed. In order to keep the pile fixation level within acceptable limits for pile design, sophisticated scour mitigation strategies are required. This paper discusses the scour protection strategy for the Nordergründe Offshore Wind Farm, where 10m seabed level drops are predicted at some monopile locations. Physical model tests were performed to validate the scour protection design, focussing on correct falling apron behaviour and the stability of the loose rock scour protection. The outcome of the tests showed good correlation with a modified relative mobility parameter. Using this parameter, the scour protection was optimised for the individual pile locations
Understanding geomorphic change on the coast over the next 100 years - the iCOASST project.
No full textUse of an agent based model and Monte Carlo analysis to estimate the effectiveness of emergency management interventions to reduce loss of life during extreme floods
No full textThis paper describes the use of an agent-based model, known as the Life Safety Model (LSM) and a Monte Carlo analysis to assess the effectiveness of emergency management interventions in terms of loss of life, taking into account uncertainties in the physical characteristics of the population at risk, represented by people’s height and mass. The work considered Canvey Island as a case study, which is located in the Thames Estuary. The average ground level of the island is 1 m below the mean high tide level. Canvey Island is protected against inundation by a series of flood defences. In 1953, the island was inundated by the Great North Sea Flood that breached the defences and led to the deaths of 58 people. A number of emergency management interventions (e.g. safe havens, flood warnings) were considered to ascertain if the loss of life in 1953 could have been reduced. The LSM was found to be an effective tool to compare different emergency management measures in order to ensure that loss of life is minimised when an extreme flood event occurs
3D Numerical modelling of pile scour with free surface profile under waves and current using the level set method in model REEF3D
Get PDFWave action stirs up sediments and keeps it in suspension while currents wash it away from the coastal zone. The combined action of the waves and current close to the sediment bed may worsen the situation by creating excessive sediment transport leading to the failure of hydraulic structures. In this study, numerical modeling of local scour under waves and current is carried out using the open source CFD model REEF3D, which solves the Navier-Stokes equation using the finite difference method. The simulated flow field from the Navier-Stokes equations is coupled with sediment transport algorithms in a numerical waves tank. Further, the calculated bedload and suspended load are linked with the Exner formula to calculate bed elevation changes. The free surface and scoured bed surface are captured using the level set method. Two case scenarios, namely scour under waves and scour under current are run until the equilibrium scour condition is achieved. The simulated results are compared with experimental data of Link (2006) and Sumer & Fredsøe (2001). Good comparison between experimental data and simulated results is observed. It is observed that for equal flow velocity in the flume, sediment transport under current only condition is larger than under waves alone
Mutual interference of bridge piers placed in staggered arrangement on scour depth
Get PDFThe work reported herein is concerned with a carefully controlled extensive experimental study of local scour around a group of bridge piers placed in staggered arrangement with constant angle of attack and varying radial pier spacing in uniform sediment at varied pier spacing under steady uniform flow clear water scour conditions at flow intensity equal to 0.95. The objective of present study is to investigate the effect of mutual interference of bridge piers placed in staggered arrangement on the scour depth. Present study reveals that the piers placed in staggered arrangement at close proximity have considerable mutual interference effect on scour depth. It is found that the downstream pier should be placed at radial pier spacing greater than six times the pier diameter since the effect of vortex shedding produced by front pier on rear pier is reasonably less
Core crack-filling by upstream gap-graded soils in zoned dams
Get PDFThe crack-filling action in zoned dams, by a granular upstream zone located upstream of a damaged core, was investigated experimentally using the Crack-Filling Erosion Test (CFET). The CFET allows testing specimens with three distinct zones: the upstream zone, the core and the downstream filter. The results of a series of laboratory tests are presented. A total of 34 tests were conducted combining 6 coarse-grained (gap-graded) upstream materials, 2 core soils, and 2 granular filters. The results of the CFETs showed that the crack-filling action is mainly controlled by some of the properties of the upstream zone and of the filter. Core soils with moderately slow erosion, or less erodible, should not have an influence on the crack-filling action. This is so mainly because the filling mechanism should occur over a very short period. The factors influencing the crack-filling by an upstream material are addressed, and some rules which give dam engineers a tool for decision-making about the potential of a upstream material to limit progression of erosion in concentrated leaks are indicated
Self-burial of objects on sandy beds by scour: A synthesis of observations
Get PDFFactors that influence equilibrium, scour-induced burial depth (B) in sand relative to object diameter (D) are examined through analysis of over 750 observations. The main factors that increase scour-induced B/D under steady currents without waves are found to be an increased Shields parameter () and small D (< 3 cm), with separate power laws applicable to clear-water and live-bed conditions. For larger D, greater cylinder density also increases B/D under steady currents. The main factor that increases scour-induced B/D under wave-dominated conditions is an increased Keulegan-Carpenter number. B/D additionally increases as the mean current component parallel to wave orbitals decreases. For cylinders under waves, B/D also increases as increases and as the angle between wave orbitals and a cylinder’s axis increases. All else being equal, tapered cylinders bury most, followed by cylinders, then spheres, and conical frustums bury least. Parameterized models dependent on the above variables explain 85% of observed variance in B/D
Surface erosion countermeasures incorporating geotextiles
Get PDFNature often demonstrates the power of water to be stronger than soil and vegetation, often even stronger than rock. Even seemingly mild hydraulic load from rainfall can be the reason for severe impact on structures and ground. To achieve erosion mitigation or to hinder erosion at all, either the action of the water has to be reduced or the resistance of the ground and man-made earth structures has to be increased. The first results in controlling, the second in opposing the water. Countermeasures to rainfall runoff are agricultural measures like disking or contour ploughing, or structural measures like terracing or building checks in erosion rills and gullies. Vegetation stabilizes the surface, but may need protection, at least during growth. Countermeasures against flood erosion at the upper bank or due to overtopping of an earth structure are appropriate reinforcement or surface protection. In all cases, well planned measures incorporating geosynthetics can help to achieve the desired stability
Physical modelling of backward erosion piping in foundation beneath levee
Get PDFCentrifuge model tests are performed to observe piping progression in foundation beneath levee and to examine influence of repeated seepage and thickness of foundation ground on piping progression. Once the pipe is formed beneath the levee, hydraulic gradient upstream of the pipe tip becomes larger while that along the pipe becomes rather small. Shift of this large hydraulic gradient position to the upstream with rise of the flood water level leads to the large subsidence of the slope in the protected side and marked increase in flow rate. Repeated seepage and thickness of the permeable foundation layer have influence on stability of levee against piping. Repeated seepage makes the piping progression faster and levee vulnerable to the piping formation. With the thinner permeable foundation layer beneath the levee, the levee is at higher risk to cause brittle failure while the required hydraulic gradient to cause piping is larger